1. Field of the Invention
This invention relates to the field of radio communication, and specifically to methods for efficient transfer of data over radio paths at frequencies below 30 MHz where available bandwidth is scarce and inconsistent propagation characteristics often seriously distort the signal. Additionally this invention relates in general to modulation and demodulation devices, more specifically, the present invention relates to a digital processing format for modulation and demodulation using an optimally shaped wave form for digital transmission of a high density band limited series of pulses.
2. Description of the Prior Art
Modems already in wide use on h.f. radio have taken one of two extremely divergent approaches in regard to the amplitude envelope the signal transmitter is required to generate. The single-tone modems produce a constant-envelope carrier and either frequency modulate it or phase modulate it with continuous functions, some of which have continuous derivatives as well. The advantage of this, of course, is that the transmitter can use a nonlinear power amplifier, and the amplifier can operate at its full power rating all the time. High frequency radio paths distort the signals in such a way that transmission of more than about 100 data bits per second on a single carrier frequently may become impossible. The standard solution is to generate many such modulated tone carriers simultaneously at closely-spaced audio frequencies and modulate them with slower-speed data streams.
Multitone modems require transmitters with linear power amplifiers because the envelope of the composite signal varies from zero to a peak which is equal to the number of tones used times the peak amplitude of each tone. As a result, the available amplifier power is used very inefficiently. The amplifier must be capable of handling the relatively infrequent enormous peaks of power, but it spends almost all of its time generating signals at a tiny fraction of its capacity. Some designs have used amplitude limiter circuits in the rf signal chain followed by bandpass filters to eliminate the out-of-band radiation produced by the clipping in a manner like speech rf "compressor" circuits. This gets the average power of the amplifier closer to its peak rating, but it also introduces data errors which the receiving processor is required to correct.